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Full text of "A Method for Determining Velocities of Saponification"

157 



A Method for Determining Velocities of Saponification. 

By Professor James Walker, r.K.S. 

(Eeceived July 10, 1906.) 
(From the Chemical Laboratory, University College, Dundee.) 

A possible method of following the progress of a reaction in which 
electrolytes are involved is to measure the electrical conductivity of the 
solution at stated intervals. This method was employed by Walker and Kay* 
in their investigation of the conversion of ammonium cyanate into urea, and 
is capable of a more extended application. 

The chief conditions for the convenient application of the method are: 
first, that there should be a considerable difference in conductivity between 
the initial and final systems, and second, that the change in conductivity 
should be proportional to the progress of the reaction. It (Kjcurred to me 
that these conditions would be well fulfilled in the saponification of an ester 
by a caustic alkali. The conductivity of the alkali, say sodium hydroxide, is 
much greater than that of the sodium salt produced by the saponification, 
owing to the high velocity of hydroxidion as compared with the salt anion. 
Since, too, sodium hydroxide and sodium salts of monobasic acids are 
approximately equally ionised under the same conditions, the ionisation in 
dilute solution remains practically the same throughout the saponification, 
and thus the alteration in the conductivity is almost exactly proportional to 
the progress of the reaction. 

The following experiment on the rate of saponification of methyl acetate by 
caustic soda may be quoted as an example of the method carried out under 
ordinary laboratory conditions with the apparatus generally used for con- 
ductivity measurements in solution. 

The conductivity cell used was of the narrow Arrhenius type, and was 
immersed in a thermostat at 25°. In this cell were placed 4 c.c. of lsr/20 
caustic soda, and 15 c.c. of water. To prevent access of carbon dioxide, the 
hole in the ebonite cover was plugged with a small rubber stopper. When 
the temperature of the thermostat had been reached, the solution was stirred 
by raising and lowering the electrodes, and the conductivity was read off. 
To the solution was then added 1 c.c. of N/5 solution of methyl acetate, the 
time being simultaneously noted, and the contents of the cell were well stirred 
by up-and-down motion of the electrodes. A reading of the conductivity was 

^ * Journ. Chem. Soc.,' vol. 71, p. 489, 1897. 



158 



Prof. J. Walker. A Method for 



[July 10,. 



at once taken after mixing, but this acted merely as a check, and was not used 
in the calculation. After a short interval readings were made of the 
conductivity of the solution (which was now IsT/lOO, both with regard to 
sodium hydroxide and to methyl acetate), at first every minute, and then every 
few minutes, when the reaction had become slower. 

The following table contains the results of the observations. In it a 
represents the bridge reading, and x the difference between the initial value of 
ajil — a)^ which is proportional to the conductivity, and the value after t 
minutes. 



t. 


al(l — a). 


J7. 


1 X 

t 1-x 



3 


(1-564) 
1-304 



-260 


0-117 


4 


1-247 


0-317 


0-116 


5 


1-198 


0-366 


0-115 


6 


1 -153 


0-411 


0-116 


. 7 


1-114 


0-450 


0-117 


8 


1-083 


0-481 


0-116 


10 


1-026 


0-536 


0-115 


12 


0-980 


0-584 


0-117 


15 


0-927 


0-637 


0-117 


18 


0-883 


0-681 


0-118 


21 


0-852 


0-712 


0-118 


25 

GO 


0-818 
(0 -564) 


-746 
(1 -000) 


0-118 


Mean -118 



It will be noticed that the constant for the bimolecular reaction is here* 
very satisfactory and in general it may be said that the maximum divergence 
from the mean value does not exceed 1 to 2 per cent. It is, however,, 
advisable to neglect the first and last fourths of the reaction, since in them 
initial disturbances and the effect of a slight departure from exact equivalence 
of the reacting substances have a relatively large effect on the value of the 
constant. 

The initial reading of the conductivity cannot be tal^en directly, owing to* 
the great speed of the reaction. In order to obtain it, the conductivity of the 
alkaline solution is read before the methyl acetate is added. In the above ^ 
instance the conductivity read in this way is that of a solation ]Sr/95 with 
regard to sodium hydroxide, 4 c.c. of N/20 solution being contained in 
19 c.c. When this is diluted to IST/lOO by the addition of 1 c.c. of the methyl 
acetate solution, the conductivity will fall off in the ratio of 100 to 95, since ^ 
the quantity of methyl acetate in the solution after mixing is less than 0*1 per 
cent., and can have no appreciable effect either on the speed or the number 
of the ions. From the conductivity of the solution before addition of the 



1906.] Determining Velocities of Saponification, 159 

methyl acetate we therefore deduct one- twentieth of its value in order to 
obtain the initial conductivity after mixing. 

The final reading may of , course be ascertained by waiting till the action 
has ceased, but it is both more expeditious and more accurate to measure the 
conductivity of a IsT/lOO solution of sodium acetate prepared by neutralising 
the ]Sr/20 solution of caustic soda by means of acetic acid and diluting to the 
requisite extent. It is well to make this measurement of the end-point 
before beginning the actual saponification. Theoretically the end-point can be 
calculated from the initial reading and the known ratio of the conductivities 
of IST/lOO solutions of sodium hydroxide and sodium acetate, but it is 
advisable to perform the experiment as a check. 

The calculation of the constant ~ . may be much simplified by the 

following device. For conductivity work a table is used which gives the 
ratio a/(l — a) for different values of the bridge-reading a. If therefore, we 
make the total range of the reaction equal to unity, we can use the same 
table to obtain the ratio xjil—x) and have then merely to divide by the 
time t, a calculation which can be performed mentally if the intervals are 
suitably chosen. Now it is always possible to make the range between the 
initial and final values of a /(I— a) equal to unity by introducing the appro- 
priate resistance in the resistance box. Thus in the above example it 
was found that the ratio between the initial and final conductivities was 
2**773 : 1. We have then i/f — 2*773, and we wish to make i—f— 1. From 
these two equations we obtain i = 1'564 and/ =0*564. We have therefore 
so to adjust the resistance that the initial value of ajil—a) shall be 1*564. 
This refers to the N/100 solution after mixing, whilst the conductivity 
actually measured is that of a N/ 9 5 solution. We must therefore increase 
the value 1*564 in the ratio of 100 to 95, and thus obtain 1*646 as the value 
of ajil—a) which the N/95 solution of caustic soda must exhibit if the 
difference between the initial and final values ©f ajil-^a) is to be unity. In 
the table this value of a/(l — a) corresponds to a = 0*622. We therefore adjust 
the resistance in the box, either by trial or by calculation, until the minimum 
for the N/95 solution of caustic soda is exactly at this part of the bridge, 
and then proceed with the experiment. This adjustment, though tedious in 
description, only occupies a few minutes in practice, and saves much time in 
the subsequent calculations. Even if readings are taken every minute, the 

1 sc 
calculation of the constant - . from each observation can be performed 

t 1—x 

in the interval between two readings, generally without the necessity of 
putting pen to paper. 



160 A Method for Determining Velocities of Saponijication. 



To obtain the velocity constant from the mean value of 



X 



it is only 



necessary to divide by the normality of the solution. In the above instance 
the velocity constant is thus 0*117-^0'01 = 11*7. 

As a further example of the method, I append a series of observations 
made by Mr. D, C. Crichton, B.Sc, on the velocity of saponification of ethyl 
acetate by caustic soda at 24°*85, the concentration of both substances 
being IsT/lOO. 



t. 


al{l — a). 


X. 


1 X 

t 1-x 





(1 -560) 







5 


1-315 


0-245 


-0649 


7 


1-247 


0-313 


0-0651 


9 


1-193 


0-367 


-0645 


11 


1-146 


0-414 


-0642 


13 


1-101 


0-459 


-0652 


15 


1-064 


0-496 


-0650 


18 


1-020 


0-540 


-0652 


20 


0-994 


0-566 


-0652 


25 


0-945 


0-615 


-0642 


27 


0-923 


-637 


-0650 


33 


0-880 


0-680 


-0644 


37 

00 


0-756 
(0 -560) 


0-704 
(1 -000) 


-0644 


Mean '0647 



The velocity constant of the saponification of ethyl acetate by caustic 
soda at this temperature is therefore 6*47, a value in exact accordance with 
the curve which expresses the results of Warder* and of Eeicherf obtained 
by the titration method. 

The conductivity method, even without special apparatus, is at least as 
accurate as the titration method carried out under specially favourable 
circumstances, and for rapid reactions is incomparably less trying in execution. 



* * Amer. Chem. Journ.,' vol. 3, p. 203, 1881. 
t ^ Liebig's Annalen,' vol. 232, p. 103, 1886.